Offset in selecting frames on the timeline
When number of frame which is selected in the Timeline / any of animation editors isn't the same as in the Current Frame value this means they are remapped to other values of Frames Per Second.
The unusual behaviour can be noticed even when clicking on frames in animation editors.



If this is not desired then change it back to default values for both Old mapping value and New mapping value (see manual about these options):


These values are 100 by default, but you can change it to any ones if they are the same.  


Can I increase the frame rate of an animation while keeping it the same length?.

How can I modify the frame rate without changing overall animation speed?.